High-power semiconductor ridge waveguide lasers suffer from the excitation of higher order modes as their power output is increased. This causes their beam shape properties to vary as a function of laser output power. Such behavior causes problems in applications which use these semiconductor ridge waveguide lasers such as optical recording and optical fibers.
There are three competing requirements on ridge waveguide lasers that are formed through wet-etching of the ridge: first, the need to have good electrical contact, and second, the need to have fundamental-mode behavior and third, the need to have low waveguide absorption. A relatively small ridge width is required for high power fundamental mode operation, and to obtain a low loss waveguide there is a need for a thick upper cladding layer which in turn increases the depth to which a ridge has to be etched in order to provide good waveguiding and reduces the contact layer width which increases series resistance or removes the contact layer completely.
U.S. Pat. No. 5,059,552 issued Oct. 22, 1991 to Harder et al., entitled: PROCESS FOR FORMING THE RIDGE STRUCTURE OF A SELF-ALIGNED SEMICONDUCTOR LASER, describes a process for forming the ridge structure of a self-aligned InP-system, double heterostructure laser, particularly useful for long wavelength devices as required for signal transmission systems that includes a thin Si.sub.3 N.sub.4 layer deposited at a high plasma excitation frequency for adhesion promotion, and a low frequency deposited Si.sub.3 N.sub.4 layer for device embedding, provides for the etch selectively required in the process step that is used to expose the contact layer to ohmic contact metallization deposition.
The publication by C. Harder, P. Buchmann and H. Meier entitled HIGH-POWER RIDGE-WAVEGUIDE AlGaAs GRIN-SCH LASER DIODE, Electronics Letters, 25th Sep. 1986, Vol. 22, No. 20, pages 1081-1082 provides background information for semiconductor ridge waveguide laser structures and fabrication methods therefor.